GROUP_CONCAT的替代方案

Question

我偶然发现了这个问题，似乎无法解决这个问题。 我有三张桌子：

MSG_V3_THREAD

MSG_V3_THREAD_USERS

MSG_V3_THREAD_PARTNERS

现在，我需要做的是最终得到这个表：

（注意：上面的图片是photoshopped）

我似乎无法找到解决方案。我设法做了一个group_concat，它给了我正确的结果，但它不是我需要的格式。我现在也有这个问题：

SELECT  msg_v3_thread.thread_id, user_id, NULL as partner_id
FROM msg_v3_thread
LEFT JOIN msg_v3_thread_users ON msg_v3_thread_users.thread_id = msg_v3_thread.thread_id 
WHERE msg_v3_thread.thread_id = 3
UNION
SELECT  msg_v3_thread.thread_id, partner_id, NULL as user_id
FROM msg_v3_thread
LEFT JOIN msg_v3_thread_partners ON msg_v3_thread_partners.thread_id = msg_v3_thread.thread_id 
WHERE msg_v3_thread.thread_id = 3

结果如下：

它只是将所有ID放在一列中。

有人可以帮我这个吗？可能解决方案很简单，但我看不到它。

谢谢！

Answer 1

你有正确的想法，你只需要切换你正在查询的显式null。除了使用union运算符的第一个查询外，所有查询都会忽略别名：

SELECT  msg_v3_thread.thread_id, user_id, NULL as partner_id
FROM msg_v3_thread
LEFT JOIN msg_v3_thread_users ON msg_v3_thread_users.thread_id = msg_v3_thread.thread_id 
WHERE msg_v3_thread.thread_id = 3
UNION
SELECT  msg_v3_thread.thread_id, NULL as user_id, partner_id -- Here!
FROM msg_v3_thread
LEFT JOIN msg_v3_thread_partners ON msg_v3_thread_partners.thread_id = msg_v3_thread.thread_id 
WHERE msg_v3_thread.thread_id = 3

Answer 2

我想你只想union这样：

select thread_id, user_id, NULL as partner_id
from MSG_V3_THREAD_USERS
where thread_id = 3
union
select thread_id, NULL, partner_id
from MSG_V3_THREAD_PARTNERS
where thread_id = 3;

请注意，union会删除表格内和表格之间的重复项。您也可以将其写为：

select distinct thread_id, user_id, NULL as partner_id
from MSG_V3_THREAD_USERS
where thread_id = 3
union all
select distinct thread_id, NULL, partner_id
from MSG_V3_THREAD_PARTNERS
where thread_id = 3;